1. Field of the Invention
The present invention relates generally to a CDMA (Code Division Multiple Access) communication system, and in particular, to a method for varying a transmission point of a dedicated channel in a CDMA communication system supporting an uplink synchronous transmission scheme (USTS).
2. Description of the Related Art
A CDMA (Code Division Multiple Access) mobile communication system is divided into a synchronous system and an asynchronous system. Such a CDMA communication system uses orthogonal codes to separate channels. Herein, a description of the invention will be made with reference to an asynchronous W-CDMA (Wideband-CDMA) communication system, also known as a UMTS (Universal Mobile Telecommunications System) communication system. However, the invention can also be applied to different CDMA systems such as the CDMA-2000 system, as well as the W-CDMA system.
The W-CDMA communication system employs an uplink synchronous transmission scheme (USTS) in which a Node B communicates with a plurality of UEs (User Equipments) through radio links formed between them while maintaining an orthogonal property among the signals received from the respective UEs. For the USTS, the Node B transmits a control signal to the UEs so that the respective UEs can transmit their signals at a proper time to maintain the orthogonal property among the UEs. Upon receipt of the control signal, the UEs align a transmission point of the uplink signals.
FIG. 1 illustrates architecture of a conventional W-CDMA communication system. As illustrated, a radio network controller (RNC) controls a process for connecting the UE. Further, the RNC manages assignment of channel resources to the UEs connected to one or more Node Bs. The Node Bs and the RNC constitute a UTRAN (UMTS Terrestrial Radio Access Network).
When successfully connected to the Node B through the channel assigned by the RNC, the UE maintains communication using the assigned downlink or uplink dedicated physical channel (DPCH). The W-CDMA communication system employs an asynchronous system in which the UEs are not synchronized with the Node B. The RNC can communication with a plurality of UEs through the Node B. In this case, the UE scrambles its transmission data using a unique scrambling code and transmits the scrambled data as an uplink signal, so that the Node B can distinguish the uplink signals received from the respective UEs.
The scrambling code is classified into a long scrambling code and a short scrambling code. In the following description, the “scrambling code” will refer to the long scrambling code.
The scrambling code is created in the following process of:
(Step 1) receiving 24 initial values n0,n1, . . . ,n23,
(Step 2) creating sequences x(i) and y(i), where i=0, . . . ,225−27,x(0)=n0, x(1)=n1, x(2)=n2, . . . , x(23)=n23, x(24)=1x(i+25)=x(i+3)+x(i) modulo 2, i=0, . . . , 225−27y(0)=y(1)=y(2)= . . . =y(23)=y(24)=1y(i+25)=y(i+3)+y(i+2)+y(i+2)+y(i) modulo 2, i=0, . . . ,225−27
(Step 3) creating a sequence z(i), where i=0, . . . ,225−2,z(i)=x(i)+y(i) modulo 2, i=0, . . . , 225−2,
(Step 4) creating a Gold sequence Z(i), where i=0, . . . ,225−2,Z(i)=1−2*z(i)
(Step 5) creating two real scrambling codes c1(i) and c2(i), where i=0, . . . ,225−2,c1(i)=Z(i)c2(i)=Z((i+16777232) modulo (225−1)),
(Step 6) creating a scrambling code C(i), where i=0, . . . , 225−2,C(i)=c1(i)*(1+j(−1)i*c2(2*└i/2┘)).
In the above formula, └i/2┘ indicates the largest one of integers less than or equal to i/2.
In the W-CDMA communication system, one frame is comprised of 38400 chips. Therefore, the scrambling code is used in a unit of 38400 chips. That is, a scrambling code for one DPCH is C(i), where i=0,1, . . . ,38399.
A DPCH frame signal is scrambled using the scrambling codes C(0) to C(38399). The respective UEs create the scrambling codes using different initial values n0,n1, . . . ,n23, and then, scramble the DPCH signals with the created scrambling codes before transmission. The Node B then descrambles the signals received from the UEs using the scrambling codes uniquely assigned to the respective UEs, thereby distinguishing the signals from the respective UEs.
The latest W-CDMA communication system uses OVSF (Orthogonal Variable Spreading Factor) codes for channel separation. In the case of the downlink, the Node B can distinguish the downlink DPCH (DL DPCH) signals transmitted to the different UEs using the OVSF codes. The Node B spreads the DL DPCH signals using the OVSF codes uniquely assigned to the respective UEs, sums up the spread DL DPCH signals, scrambles the summed DL DPCH signal with its unique scrambling code, and then transmits the scrambled DL DPCH signal. The respective DPCHs may have different data rates. In the case of the uplink, the UE spreads a DPDCH (Dedicated Physical Data CHannel) signal and a DPCCH (Dedicated Physical Control CHannel) signal constituting a DPCH signal, using different OVSF codes, and scrambles the spread DPDCH and DPCCH signals with its unique scrambling code before transmission. The OVSF codes used by the UE to spread the DPDCH and DPCCH signals may also be identical to each other. Since the UEs transmit the signals using the different scrambling codes, the Node B can distinguish the signals received from the respective UEs.
The UE employing the USTS scrambles the DPDCH signal and the DPCCH signal spread with the different OVSF codes using an uplink scrambling code commonly used by the UEs in a cell where it is located, instead of using its unique scrambling code, and transmits the scrambled signals. Further, the UE employing the USTS spreads DPDCH signal and the DPCCH signal with a unique OVSF code, i.e., a channelization code, assigned from the Node B, and transmits the spread signals. The Node B then despreads the signals received from the respective UEs using the OVSF codes uniquely assigned to the UEs, thereby distinguishing the received signals.
In addition, the W-CDMA communication system transmits the respective DL DPCH signals with different time offsets, in order to prevent the transmission power from increasing instantaneously when the Node B simultaneously transmits a plurality of downlink DPCH (DL DPCH) signals. By doing so, the uplink DPCH (UL DPCH) signals also arrive at the Node B at different points in time, preventing the Node B from simultaneously processing the signals received from a plurality of UEs, thereby distributing a load of the Node B.
FIG. 2 illustrates the timing relationship between the DL DPCH signal and the UL DPCH signal in the W-CDMA communication system, wherein it is assumed that there is no propagation delay between the Node B and the UEs, i.e., that the UE receives the DL DPCH transmitted by the Node B with no propagation delay and the Node B also receives the UP DPCH transmitted by the UE with no propagation delay. When there exists a propagation delay between the Node B and the UEs, a round trip time (RTT) must be considered. However, since the system will operate in the same manner even though there exists the propagation delay, the round trip time will be assumed to be ‘0’.
Referring to FIG. 2, one 10-ms frame is comprised of 15 slots, and each slot is comprised of 2560 chips. A common pilot channel (CPICH) and a primary common control physical channel (P-CCPCH) are frame-synchronized with each other, and used as a reference time for other channels.
As illustrated in FIG. 2, the respective DL DPCHs are transmitted with a time offset τDPCH,n from the P-CCPCH. The respective DPCHs are given the different time offsets τDPCH. For example, each DPCH is given one of 0, 256, 2*256, . . . , 148*256 and 149*256-chip offsets. That is, the DPCH is given a time offset of a multiple of 256 chips from the reference time.
The UE transmits the UL DPCH after a lapse of a To time after receiving the DL DPCH with a time offset τDPCH,n from the P-CCPCH. Therefore, the UL DPCHs also have different transmission points, so that the UL DPCH signals arrive at the Node B at the different time points. Due to a distance difference between the Node B and the respective UEs, the Node B may not receive the UL DPCH exactly after a lapse of the To time after transmitting the DL DPCH. Therefore, the Node B measures a propagation delay time to the UE in the process of transmitting a random access channel (RACH) in order to measure a distance from the UE, and uses this value in predicting an expected UL DPCH arrival time after transmission of the DL DPCH.
In the USTS mode, a plurality of UEs can communicate with a Node B using the same scrambling code. The USTS is designed to synchronize the UL DPCHs received at the Node B from a plurality of UEs. In the USTS mode, the Node B can assign the same scrambling code to the synchronized UEs. Therefore, the W-CDMA communication system employing the USTS can reduce the number of scrambling codes used in the cell, contributing to a reduction in interference between UE signals. When the UEs employing the USTS use the same scrambling code, the Node B can identify the UEs using channelization codes, i.e., the OVSF codes provided from the RNC. In the USTS mode, the Node B synchronizes the UL DPCHs from at least 2 UEs with each other, and then assigns the same scrambling code to the synchronized UEs. Further, the Node B assigns the different channelization codes (or OVSF codes) to the UL DPCHs of the UEs assigned the same scrambling code, to distinguish the received synchronized UL DPCHs.
The USTS controls a sync time of the signal through the following two processes.
(1) Initial Synchronization Process
Upon receipt of a signal from a UE over the RACH, a Node B measures the difference (offset) between a predetermined reference time and an arrival time of the signal received over the RACH. The Node B transmits the measured time difference to the UE over a forward access channel (FACH). Upon receipt of the time difference over the FACH, the UE aligns its transmission point using the received time difference.
(2) Tracking Process
The Node B transmits a time alignment bit (TAB) to the UE by periodically comparing the arrival time of the UE signal and the reference time. If the TAB is ‘1’, the UE shifts the transmission point ahead by ⅛ chip. However, if the TAB is ‘0’, the UE shifts the transmission point behind by ⅛ chip. The TAB is transmitted once every two frames using a transmit power control (TPC) bit in the control channel.
In the USTS mode where several UEs use the same scrambling code, the uplink frame signals transmitted by the UEs using the same scrambling code must be synchronized with one another at the Node B. That is, when the Node B receives the DPCHs transmitted from several UEs, the received DPCHs must be subjected to both slot synchronization and frame synchronization. The frame synchronization is to minimize interference among the UEs using the same scrambling code, while the slot synchronization is to distinguish the UE signals from each other. The UEs perform spreading using the different OVSF codes and perform scrambling using the same scrambling code, depending on the orthogonal property of the OVSF codes. The Initial Synchronization Process is a process for acquiring the frame synchronization and the slot synchronization.
As described above, the respective DL DPCHs have different time offsets τDPCH,n. Therefore, the UL DPCHs received at the Node B are not synchronized (or misaligned) with one another. During the Initial Synchronization Process, the misalignment among the UL DPCHs is removed to synchronize the UL DPCHs. Accordingly, there is a demand for a method for resolving the channel misalignment problem in the Initial Synchronization Process.
As stated above, since the USTS synchronizes the uplink within one cell and uses the channelization codes and a specific scrambling code different from a normal DPCH not supporting the USTS service, a special handover method is required. That is, in the case of the normal DPCH, each UE uses a unique uplink scrambling code. However, in the case of the USTS, a plurality of the UEs share the same scrambling code. Further, in the case of the normal DPCH, a node position of an OVSF code for spreading the DPCCH signal is SF256 which is the highest position in the OVSF code tree. However, in the case of the USTS, the node position may be different. In addition, a node position of the OVSF code for spreading the DPDCH may also be different from the node position of the OVSF code in the OVSF code tree, used by the normal DPCH. However, in the case of the USTS, the UE performs special synchronization, so that when the handover is performed in the same method as done by the existing UMTS system, two or more connections operate differently. Therefore, it is not possible to perform a USTS handover using the existing handover method. Thus, there is a demand for a separate handover method.
In addition, there is a demand for a mode switching method for switching an operation mode of the UE to the USTS mode, when the UE, which supports the USTS service but operates in the normal mode or non-USTS mode, enters into coverage of the Node B supporting the USTS service.
Furthermore, it is necessary to control the timing of a signal transmitted from a second Node B to a UE, when the UE moves to the second Node B while communicating with a first Node B employing the USTS, i.e., when the UE transitions from one state where the UE communicates with the first Node B in the USTS node and with the second Node B in the non-USTS mode to another state where the UE communicates with the second Node B in the USTS mode.